I have often heard that SSTO is too hard and we should be thinking about 2STO instead. After all, if the Armadillo or DC-X or RVT or whatever can take passengers sub orbital and land under computer control, it could take a second stage up instead of passengers. So maybe a sub orbital vehicle big enough to take up 50 people would be big enough to take up a second stage that could itself accommodate maybe 5 people. There would be extra operational costs associated with bringing the first stage back to the launch site, since it would land far down range, but that may be offset by the lower maintenance costs. Two lower mass fraction vehicles should more robust than one very high mass fraction vehicle. If the sub orbital market proves good enough, a 50 passenger sub orbital vehicle may be built anyway. It would then be a much smaller job to adapt it for use as a first stage.

Or…

I have also heard many times on this board that air launch is not worth it for orbital flight because of the DeltaV needed is so high. But I did some Excel numbers for a rocket with isp of 450, which is achievable for LH2/LOX engines. Assume a DeltaV of 9.7 is needed to get to orbit. That is 7.8 for orbital speed plus 1.9 for air drag and gravity loss. An SSTO rocket needs a mass fraction of about 10.5 to do that. It is difficult but maybe not impossible to achieve. Now if it was air launched at only 1 km/s, which is about mach 3, then a rocket with a mass fraction of only 8 could supply the other 8.7 km/s. And depending on the launch altitude, quite a bit of air drag would be avoided, and some gravity drag too, so maybe a DeltaV of less than 8.7 would do, further reducing the mass fraction needed. A mass fraction of less than 8 is definitely achievable. I believe the Japanese are working on a Mach 3 airliner for trans-Pacific service. Building on this work I believe a Mach 3 aircraft with short range but high payload capability would possible. Not knowing aerodynamics, I would think the high speed separation would present control problems, but I bet that it could be managed by people who DO know aerodynamics. This idea is something like the first space shuttle plans, which called for two winged vehicles, a large supersonic launch aircraft and a small rocket plane carried piggy back on it. Maybe it is time to revisit that architecture but using the new materials and engines available today. And Burt Rutan is just the man to do it too.

A mach 3 aircraft costs much, much, much more to develop than a mach 3 rocket. I really don't think we will see any commercial launches from supersonic aircraft (there was an ASAT test from a fighter jet at one point). While there have been proposals on those lines in the past, the original shuttle design wasn't one of them. The booster was a conventional vertically launched rocket that just had wings for recovery (still a bad ideo, IMHO).

The primary benefit of air launch isn't the delta-V anyway, it is potential safety benefits, and putting the rocket up where the atmosphere is thinner so a higher expansion nozzle can be used.

theoretically, if done right, a subsonic air launch from about as high as a large cargo plane will go (say 40000ft) will provide almost 1km/s in delta-V benefit as compared to a surface launch of a "normal" rocket, say an Ariane or Atlas. This is, roughly speaking, composed as follows:
- The velocity of the airplane itself is about 250 m/s.
- The gravity loss a normal rocket incurs until it has reached this altitude and speed may be another 300 m/s or so if my hand calculation was right (remember it's travelling slowly at the beginning, so gravity loss is important in the early flight phases). Attention: Gravity loss depends a lot on thrust/weight ratio in the early (vertical) flight phase because it depends on the time you thrust vertically, so it may vary between concepts.
- The better nozzle geometry is responsible for an almost similar amount, though obviously hard to generalize (if you take a Vulcain engine, the Isp is about 431s in vacuum and I think 340s at sea level, so make your own calculation).
- Some non-negligible parts are also provided by the potential energy at that altitude, and the reduction in atmospheric drag.

On the other hand, there are some drawbacks: The vehicle needs to be fairly sturdy (most traditional rockets would simply break apart when dropped unceremoniously like the AirLaunch one, they're just so optimised). That adds a little structural mass. And when you jettison, there's typically a penalty because you're not necessarily flying in the speed and direction you need. Pegasus therefore uses its wings for correction, and AirLaunch simply gives up some velocity. Ideally, you'd be launched while the carrier aircraft does a pull-up, but try that with a C-17 or C-5 at high altitude!

Finally, two percent more dead mass available makes all the difference in a rocket. Just compare the structural mass of various launchers and you'll see they're all pretty close together! So, that 1km/s or so is really helpfu. OK, you might get another 600 m/s or so travelling at Mach 3, but you'll have spent a few billion up-front first.

Best regards
Max Lange

_________________There's space for all of us, if each will leave some space for the next one

The ideas expressed above are my own, not necessarily those of my employer.

While there have been proposals on those lines in the past, the original shuttle design wasn't one of them.

Yeah, I am talking about before the shuttle program. There were ideas around in the 1960's about a hypersonic turbojet/ramjet aircraft that would carry a smaller rocket plane on it's back. Estes even made a model from the idea.
http://www.washingtonhighpower.com/BARPICS/OT.jpgBut I agree, a rocket to get the same speed is simpler. That is why I was assuming a rocket first stage would provide at least 3 km/s but allowed only 1 km/s for the aircraft. I think the airplane idea may still have a place, for someone like Rutan who likes airplanes, but I am also looking forward to Armadillo making a computer controlled fly back reusable first stage.

[quote="spacecowboy"]The tandem-boosters concept seems kinda awkward (but very doable), and the piggyback is just plain sexy. I actually kinda like 'em.[/quote]

Naah, it's undoable, or at least way suboptimal. The aerodynamics are a nightmare and the highest mechanical stress is at the narrow junction. The whole thing seems to me like a typical NASA "give us billions and we'll make something that we're not sure has a market, and will spend a good part on the way on technologies we don't need for it " programme.

A good TSTO launcher won't have a winged upper stage - not worth the bother. It'll respect basic engineering principles. And it won't need a major technological breakthrough that's always a few years ahead. Proof? Look round, man! Or ask Elon Musk.

Cheers anyway Max

_________________There's space for all of us, if each will leave some space for the next one

The ideas expressed above are my own, not necessarily those of my employer.

I'm fed up with things that look cool but never make it off of a graphic designer's computer screen, that cost billions and take years before they are cancelled...

Well spoken!

Apply new technologies one or two at a time, after you've learned to master them somewhere else, and it'll look like a big breakthrough twenty years later. Doing the possible is what brings results. But I'm repeating myself...

G'night
Max

_________________There's space for all of us, if each will leave some space for the next one

The ideas expressed above are my own, not necessarily those of my employer.

I agree. At least, the ESC-A has a similar recurring cost to the Ariane-44L which carried half the payload. That's nice incremental progress.

The whole Ariane-5 business is an example how launcher development was turned away from big-government-sponsored cost-plus "get me a mini shuttle launcher" to a business-oriented model. These days it's all fixed firm price contracts, and Arianespace is a commercial entity. That's been quite a learning process for Europe (and OK, under the surface it's still going on, but it's come a long way).

Talk to you later
Max

_________________There's space for all of us, if each will leave some space for the next one

The ideas expressed above are my own, not necessarily those of my employer.

Oh, come on, we can hope for ANYTHING... but we would be amiss to expect more than incremental improvements in almost all cases.

The RASC folks have put together a nice presentation, and I for one am in favor of air-breathing, lifting-airframe, fully re-usable hypersonic TSTO technology; and I believe that it really may become practicable before the elevator concept puts it to rest (excepting possibly for extraterrestrial applications).

But the list on the final page of "enabling technologies" which need to be developed is frightfully sobering. Burt Rutan probably has the drive and the engineering acumen to clear those hurdles, but he'll need lots of time and funding. And his plate seems kinda full already.

I think we will continue to see air-launch strategies from MAV, and we may even see high-altitude supersonic air-launch at some point, but indications at this time seem to imply that Rutan feels the optimal near-term solution lies in simple rockets launched from intermediate altitudes on subsonic aricraft. Given that he has a reputation as a minimalist engineer, I think that he intends to fulfill his own dream of moonwalking using something more like the t/Space model.

But I would be just tickled if he rolled out an SS3 that looked like HyperX crossed with an SR-71.

I for one am in favor of air-breathing, lifting-airframe, fully re-usable hypersonic TSTO technology

Yes, the winged, fully reusable, airport operating, low mass fraction ideas are attractive, and in the very long run will probably be the way to go. But I have to agree with John Carmack that we can expect VTVL two stage fully reusable rockets first.

I agree. At least, the ESC-A has a similar recurring cost to the Ariane-44L which carried half the payload. That's nice incremental progress.

The whole Ariane-5 business is an example how launcher development was turned away from big-government-sponsored cost-plus "get me a mini shuttle launcher" to a business-oriented model.

But they had to have the gov't push first. People thaought that Ariane 5 was 'too big' to be marketable, and were proved wrong when the recurring cost proved no different than Ariane 4. Human spaceflight has been a force driver. First with R-7 and Gagarin, then with Ariane 5, and later with HLLV reducing the need for many Delta IV flights. The Ariane 5 was to be a man-rated launch vehicle, so extra care was given.

Hermes is gone, but the wisdom of the bigger launch vehicle remains.

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The only thing that wories me about VTHL TSTO is that you will have twice the wing weight, twice the landing gear, etc. Both boosters need to be able to land on their bellies and launch on their tails. A SSTO design might be simpler. StarBooster's Hu Davis has been trying to get a simple fly-back booster but has been ignored. Branson turned Hu down flat for Rutan, even though Hu was part of the Apollo generation and knows his business. Sadly, even Pete Worden seems to have abandoned Hu.

But they had to have the gov't push first. People thaought that Ariane 5 was 'too big' to be marketable, and were proved wrong when the recurring cost proved no different than Ariane 4. Human spaceflight has been a force driver. (...)Hermes is gone, but the wisdom of the bigger launch vehicle remains.(...)

Hello Publius,

you're right. Ariane 5 needed quite a lot of gov't push. For a launcher that size, until now that has been the only way. My point is, though, that the way it is now made, eschewing cost-plus contracts for firm fixed price, is a step towards market thinking. That helped to keep the recurring cost down, by reducing wasted efforts.
Talking about bigger launch vehicles, Ariane 5 also has some difficulties. GEO satellites aren't so plenty these days, so it often has to wait in order to match payloads. That reduces the launch rate. A flexible system, like Ariane 4, would be more advantageous. OTOH, a super-Ariane might offer x times the payload for y<x times the price, but the x/y ratio needs to be fairly large in order to open a new market. You always need to consider the total investment of the possible customer.

Of course, with JSF dollars, you could do something along that line. But would it be sensible? I'd almost rather let the emerging heavy-lift market drive development, rather than getting another white elephant.

Regards
Max

_________________There's space for all of us, if each will leave some space for the next one

The ideas expressed above are my own, not necessarily those of my employer.